WMG / Doraemon

Sewashi will have a kid that looks like Shizuka but with glasses

• In a future special or a future movie (after 2019's movie), Sewashi's daughter will travel to the past to meet Nobita. She will be named after and reminded to Nobita of Lulli due to her blue hair and eyes.

Shizuka will go on a pilgrimage

• She will go on a pilgrimage to find her ancestors (way before Shizu) known as the People of the Time remains and a lost dialect of Japan in Siberia in a future movie.

Doraemon, no, not just him, but the entire Doraemonverse is also a large SCP designed specifically to content him, with the prehistoric populations being clones or mind-wiped D-class

Given that the series runs on Negative Continuity at times, how past, present, and future is constantly static, and how Doraemon is not only having potentially reality warping gadgets, but also an Unusually Uninteresting Sight despite his obvious appearance... The so-called "Time Patrol" is actually a squadron of MTF to keep the Doraemonverse from ever collapsing (or knowing that the universe isn't "prime") and to ensure the safety and containment of Doraemon and friends.

Sewashi isn't actually Nobita's descendant

He could be a genetic clone (no surprise given the sci-fi theme), or he could simply be adopted; nevertheless he is sure even if the past (Nobita's present) is changed he won't be erased from the existence.

• Alternatively, he's the descendant of one of Nobita's cousins and Nobita's grandchildren just adopt him. Nobisuke (Nobita's dad) has one brother and several cousins, so it's not hard to imagine that Sewashi could be descended from one of them. Fridge Horror kicks in once you realize that Sewashi knows full well that his adopted parents will be erased from existence, but do it anyway to get a better life.

The Gang de-ages themselves at the end of each movie adventure

The gang have been though over a thousand episodes' worth of anime, and several dozens of films, and they're still the same age as they were back in their debut.

Out-of-Universe, it's because they're Not Allowed to Grow Up. But there's actually an in-universe explanation that was briefly mentioned in Doraemon: Nobita's Little Space War, when Suneo wonders if they could make it back home before dinnertime after going to space, with Doraemon saying they can use the Time Machine in Nobita's bedroom and return to the precise moment they depart.

Given that Doraemon has a Time Cloth gadget that can de-age humans (demonstrated in the manga short "Nobita Spends 3000 Days on a Deserted Island" to turn Nobita from 20 to 10), they probably use this trick for adventures that spans the course of several days, or even weeks (like their escapades in Bauwan, Chamocha, Aquaria, Pokkoru etc).

Heck, Nobita and friends probably have a mental age of 30 or so, despite looking like children, because of all the adventures they've been through...

Nobita with Doraemon's intervention is even wimpier than the original Nobita

Left without Doraemon's gadgets to fall back on, the original Nobita will probably get himself together at one point (since he actually manages to get a job). Also, since it's revealed that Shizuka marries our Nobita out of sheer pity after a mountain hike gone wrong, it's possible that she doesn't marry the original Nobita because he isn't as pathetic.